Process for modifying cellulosic materials and product thereof



RUNNING TIME (HOURS) TO 75% ORIGINAL TENSILE STRENGTH RETENTION Nov. 6, 1962 T. RAPHAEL ET AL PROCESS FOR MODIFYING CELLULOSIC MATERIALS AND PRODUCT THEREOF Filed June 28, 1960 E I00 2 3: 9o I- -cr E 80 Ed 0) gu l 7O '5 so B- 12 2 5g 50 4 I I a I I J O o 2 4 e a lo I2 l4 l6 ACCELERATED AGING TIME, HOURS I- UNTREATED III-UREA-FORMALDEHYDE TREATMENT III-PHENOLIC RESIN TREATMENT III-COMBINATION PHENOLIC AND I0,000; UREA-FORMALDEHYDE RESINS TREATMENT 1- COMBINATION PHENOLIC AND UREA-FORMALDEHYDE RESINS WITH UREA TREATMENT IOOO;

IO: o -l "83 9 2 IOI up |2 o |g |4 2 I5 4 I6 |7 9 l9 3 g a OPERATING INVENTORS TEMPERATURE THOMAS RAPHAEL RICHARD E. MERRILL WILLIAM WEBB, JR.

United States Patent 3,062,699 PROCESS FOR MQDEFYKNG CELLULOSIC MATE- RIALS AND PRODUCT THEREOF Thomas Raphael, Winchester, and Richard E. Merrill, Wakefield, Mass., and William Webb, J12, Milton, N.H., assignors, by direct and mesne assignments, to fipanlding Fibre Company, Inc, Tonawanda, N.Y., a corporation of New Hampshire Filed June 28, 1960, Ser. No. 39,347 Claims. (Cl. 162135) This invention relates to cellulosic products and process and more particularly to a paper having low hydroexpansivity and improved resistance to heat.

It is customary to use a paper product, usually in the form of strips, as insulation in electrical equipment which in turn is operated in oil. For example, so-called transformer boards are used in oil-cooled electrical power transformers which operate continuously at or rise intermittently to elevated temperatures. Under these conditions cellulosic materials deteriorate by a dehydration reaction when subjected to elevated temperatures for prolonged periods. Moreover, the cooling oil tends to be come acidic in nature with prolonged operation of the electrical apparatus, and the acid conditions to which the cellulosic material is exposed contribute to its deterioration.

In the construction of such electrical equipment, for example transformers, it is necessary to attain close tolerances to obtain a tight package. In assembling, wire coils must be wound tightly in order to keep them from moving under the magnetic field forces of operation. If the transformer board used is particularly dry when it is cut to size, and the windings turned on it, the board will subsequently expand when exposed to normal or humid air and a unit may not fit in the transformer shell. If, on the other hand, the boards are moist when they are first wound with the wire, they may contract and leave the wire loosely fitting.

Finally, demands have recently increased for miniaturized electrical equipment which in turn has resulted in the demand for increasing the operating temperature of the equipment due to the poorer cooling inherent in the smaller units. This in turn indicates the need for cellulosic material and particularly for transformer boards and other insulation which can stand high current usage and thus higher temperatures.

It will be seen from the above discussion of the problems involved in using cellulosic material in oil-operated electrical equipment that it would be highly desirable to have a cellulosic material which exhibits improved resistance to acids and to higher temperatures and which at the same time can show improved dimensional stability by virtue of being less hygroexpansive. In the following description and discussion, the term paper" will be used in a broad sense to indicate sheets formed of cellulosic fibers. The sheets themselves may vary in thickness from normal paper to relatively thick paperboard.

It is therefore an object of this invention to provide a cellulosic product or paper having improved thermal stability. It is another object to provide paper of the character described which exhibits improved thermal stability in the presence of oil which may or may not be acidic. Another object of thisinvention is to provide a modified cellulosic product which has improved dimensional stability with respect to varying moisture conditions, that is, a cellulosic product which exhibits low hygroexpansivity compared to other cellulosic materials. It is yet another object to provide paper of the character described which while exhibiting good thermal and dimensional stability yet is not brittle so that it can be bent or formed or otherwise fabricated for assembly. It is another object to pro- Patented Nov. 6, 1962 vide a paper suitable for installation in electrical apparatus which will permit the apparatus to operate at temperatures higher than those now permitted or which will permit the apparatus to operate over longer periods of time than are now possible.

Another object of this invention is to provide a process for forming a paper having improved thermal and dimensional stability and a high degree of flexibility. These and other objects will become apparent in the following description.

The modified cellulosic product of this invention may be characterized as one which contains from 5 to 30% by fiber weight of a phenolic resin, from 2 to 10% by weight of a urea-formaldehyde resin and from 1 to 7% of urea, the urea-formaldehyde resin and urea preferably being present in substantially equivalent amounts and the phenolic resin being present in a concentration equivalent to from about 1 to 5 times the amount of urea-formaldehyde resin or urea. However, somewhat more ureaformaldehyde resin than urea is permitted within these ranges as specified. A small amount of an elastomeric substance may also be introduced into the paper to give it an added degree of flexibility.

The process of this invention may be characterized as treating a cellulosic material to render it dimensionally stable and resistant to heat, characterized by the step of incorporating into sheets formed of cellulosic fibers from 5 to 30% by fiber Weight of the phenolic resin, from 2 to 10% of urea-formaldehyde resin and from 1 to 7% of urea introduced in the form of a concentrated water solution preferably before the resins are cured.

Within the range specified it is possible to make a paper product having very low hygroexpansivity and a life expectancy of some 12 times greater than an untreated board when it is exposed to oil temperatures of about C., the present temperature limit placed on transformer operating temperatures. For an equivalent life expectancy of for example 4500 hours, paperboard made in accordance with this invention increases the allowable operating temperature from about 105 C. to about C. That these improvements cannot be attained by adding any one of the agents used, or a combination of any two of them, will be clearly illustrated in the descrip tion below. It will be shown moreover that the combination of the three components used in treating the cellulose in the range specified achieves a true synergistic effect with respect to improvements in thermal stability and dimen sional stability.

The invention may be further described in detail with reference to the following drawings in which FIG. 1 is a plot of aging time against percent retention of tensile strength for typical transformers boards which are untreated and which have been treated wtih single resins, with a resin mixture, and according to the teaching of this invention; and

FIG. 2 is a plot of life expectancy versus temperature for insulating boards which are untreated and treated according to this invention.

In order to better describe the invention and to discuss the plots of FIGS. 1 and 2, there is given first an example embodying the process of this invention.

Example I In making the paperboard of this example a typical phenolic resin and a typical urea-formaldehyde resin were used. To form the phenolic resin from 1.1 to 1.15 moles of formaldehyde (as 40% formalin) was added to one mole of phenol, and then 12 to 15 grams of concentrated ammonia was mixed in. This composition was refluxed for 50 minutes and the resin formed was then discharged from the refluxing kettle onto a cold floor. The cooled 3 resin was broken into lumps and ground into finely divided particles, care being taken not to overheat the material during grinding to prevent advancing the resin to its cured state.

The urea-formaldehyde resin was made by condensing 30 parts by weight urea by refluxing for a short time with 100 parts by weight neutral 30% formaldehyde. Five parts by weight boric acid was dissolved in a small amount of water and added to the urea-formaldehyde mixture which was then boiled for a further period. After two and onehalf hours the reaction mixture had become converted into a moderately viscous solution which did not become cloudy on cooling, this test marking the polymerization of the initially formed methylol urea material into a colloidal hydrophilic sol. This was concentrated somewhat by evaporation.

A cellulose fiber slurry of 0.05% consistency was made by beating rag and kraft fibers in a ratio of 2 to 8 parts by weight until a suitable papermaking stock had been formed. Into this slurry was then introduced sufficient quantity of the powdered phenolic resin made as described above to supply 15% by weight of the fiber and sufiicient of the urea-resin suspension to supply 5% by weight of the fiber. The slurry was then adjusted to a pH of 8 to 9 with NH OH and then an elastomeric material (in this case a 40% solids concentration of an acrylonitrile-butadiene latex) was also added to the slurry in an amount to provide about 1% of the latex by dry fiber weight. After thorough mixing sufiicient papermakers alum and acetic acid were added to reduce the pH to 4.5 to 5.0 and to exhaust out the resins and elastomer onto the fibers, and the paper stock was then sheeted out -to form a paper on a paper machine cylinder. Onto the Wet fiber web thus formed on the cylinder was sprayed a concentrated water solution (about 50%) of urea, sufiicient to incorporate into the paper about 5.0% by weight of urea. The treated paper was then subsequently dried by normal paper drying techniques and subsequently cured by baking for two hours at 350 to 400 F.

For comparison papers were made in the same manner as described except that one received no resin or urea treatment, another was treated with the phenolic resin to the extent that 15% resin was present in the paper, a third was threaded with urea-formaldehyde to the extent that 5% of it was present, while a fourth was treated with a combination of phenolic resin and urea-formaldehyde resin in an amount equivalent to 15% and 5%, respectively, by fiber weight.

The paper made in accordance with this invention as described in this example and the four papers formed for comparison were then evaluated to determine the percent of their original tensile strength which .was retained with increasing aging times. These evaluations were carried out in accordance with the testing procedure described in a publication of papers presented at the AIEE 1954 Winter General Meeting entitled Thermal Evaluation of Insulating Materials published by the American Institute of Electrical Engineers.

The result of the determination of percent retention of tensile strength with aging time in hours is plotted in FIG. 1. It will be seen in this figure that treating the paper with only one of the treating components required by this invention or with a combination of two of them does not result in the thermal stability shown by the paper treated in accordance with this invention. Perhaps the best way to illustrate this is to assume that the paper or transformer board, as the case may be, must retain at least 75% of its original tensile strength to be usable as an insulation material continuously exposed to an oil bath. FIG. 1 shows then that under these accelerated testing conditions which gave the measurements for plotting FIG. 1, an untreated board (curve I) or a board receiving urea-formaldehyde resin treatment (curve II) would fall below the required 75% of its tensile strength after only two and one-half hours. A board receiving only a phenolic resin treatment (curve III) under the same conditions would reach tensile strength in five hours, while one which received a combination treatment with phenolic resins and urea-formaldehyde resin (curve IV) would have its useful life under these conditions extended to somewhat less than 7 hours. In contrast to this, a paperboard treated in accordance with this invention (curve V) shows that it would reach 75 tensile strength after 12 hours. This is almost twice that of the best performance achieved by the other treatments noted.

It is possible to extrapolate the data of FIG. 1 to plot FIG. 2, which represents the expected life expectancy of a paperboard under normal operating conditions. The life of such a material is considered to be terminated when the tensile strength falls below 75% of its original tensile strength. This life expectancy is plotted as the logarithm of time in hours against the reciprocal absolute temperature expressed as the equivalent operating temperatures in C. Assuming for example that the maximum permissible temperature was about C. it will be seen from FIG. 2 that the untreated board (curve I) can be expected to have a life expectancy of approximately 4500 hours; while a board treated in accordance with this invention (curve V) would have a life expectancy at the same operating temperature of close to 55,000 hours, or some 12 times as long. Another Way of comparing the performance of the two types of boards is to assume a fixed life expectancy, say of 4500 hours, and to show that under these conditions the untreated board could operate at 105 C., while the treated board could operate at C., thus attaining the desired thermal stability at higher temperatures which was indicated to be desirable in the demand for miniaturized equipment.

The data plotted in FIGS. 1 and 2 illustrate the marked improvement in thermal stability that is achieved by the combination of treating agents used. Dimensional stability measurements were also made as described below.

The increases in dimensions of a bone dry sample sheet measuring 4x 4 inches and 4; inch thick when exposed to 90% relative humidity until the moisture content was in equilibrium with the surrounding conditions were measured for a first sheet prepared in accordance with this invention and for a similar second control sheet which was untreated. The increases in dimensions are tabulated below.

This shows that the treatment with a combination of a phenolic resin, a urea-formaldehyde resin and urea reduces hygroexpansivity in linear dimensions from 15 to 25% below that exhibited by a typical, commercially available paperboard used in transformers. Even more significant is the fact that changes in thickness of a paperboard can be reduced to about one-half by the treatment of this invention. Thus dimensional stability is also improved by this treatment and by the resulting cellulosic product.

In defining the treating agents suitable for the practice of this invention and in attempting to understand how the combination of them achieves the desired results, it Will be helpful to point out what is believed to take place when a cellulosic material is exposed to oil at elevated temperatures for a prolonged period. The oil to which the cellulosic material is constantly being exposed has acidic constituents and under normal circumstances these act autocatalytically on the cellulosic fibers to break them down and once the reaction has started the cellulosic material deteriorates extremely rapidly. However, it

appears that the degree of acidity of the oil or the temperature at which it is maintained are not separable factors, for an oil can be acid to some extent without affecting paper, or conversely cellulosic materials can be degraded by heat alone. There are treatments that can be used to keep the oil nonacidic but which do not materially retard thermal degradation of cellulosic materials on it. Thus, it would appear that there is established a peculiar situation, and that a combination of effects are necessary to achieve both resistance to heat and acid. As shown below, the components making up the treatment of this invention achieve this combination of effects.

The phenolic resins which are suitable for this invention are those which normally fall within the accepted definition of phenolic resins and include the condensation product of formaldehyde with phenol, resorcinol, cresol, xylenol and other mono and di-hydroxy phenols. The preferred form of phenolic resin is one which is generally designated as a resole, a term used to connote single stage resins. A typical preparation of such a phenolic resin has been illustrated in the example above. However, they may be prepared in other ways according to methods well known in the resin art. The phenolic resins may be added as the finely divided solid materials such as in Example I or they may be added as water solutions (A-stage resins) or as water dispersions, depending upon the degree to which the condensation has proceeded and the manner in which the resin was originally formulated. The phenolic resin may also of course be a mixture of phenolic resins, e.g., a phenol-formaldehyde and cresol-formaldehyde resin mixture. The phenolic resin should be present in the final treated paper in a quantity equivalent to from 5 to 30% by dry fiber weight. It may be added at any step in the papermaking process up to the point where the paper stock is introduced into the papermaking machine.

As has been noted above in connection with FIG. 1 in Example I, phenolic resins do have some salutary eifect on thermal stability of a cellulosic product. However, as also pointed out above, it in no way achieves the improvement in thermal stability achieved by the combination of treating agents required. It is believed that phenolic resins are capable of reacting with certain portions of the cellulosic molecule to prevent further oxidation and that they are also capable of contributing some dimensional stability to a paper which contains them. The phenolic resins alford their greatest protection in the early stages ofthe heating history but finally lose their protective action.

The urea-formaldehyde resins are those which are known in the art and as in the case of the phenolic resins, several methods exist for their preparation, a typical one being given in Example I above. It should be pointed out that other amine materials may be used in forming the condensation product with formaldehyde and these include melamine and thiourea. Mixtures of resins made from these amine materials with formaldehyde may also be used.

The urea-formaldehyde resin is conveniently added as a water solution or suspension. As in the case of the phenolic resins the urea-formaldehyde resins may be added at any point in the papermaking process up to the point where the stock is introduced in the papermaking machine. The urea-formaldehyde resins will be added in amounts sufficient to provide from 2 to by weight urea-formaldehyde in the finished paper.

FIG. 1 illustrates that to treat a cellulosic paper with a urea-formaldehyde resin alone does not contribute materially to the thermal stability of the board, at least if it is desired to retain at least 75% of the original tensile strength of the paper. The urea resin does not contribute protection in the early stages of the heating history but does prevent degradation from continuing over the long run. However, it appears that in using it in the process of this invention in conjunction with phenolic resins and urea, the urea-formaldehyde resin is capable of con tributing to the thermal stability of the resulting paper. It is also possible to assume that the urea-formaldehyde resins are capable of reacting with those portions of the cellulosic molecules which are unattacked or unreacted with the phenolic resins. Thus, the urea-fonnaldehyde resin and the phenolic resins are not interchangeable but are complementary in their effect on the cellulosic material.

The third treating agent of this invention is urea, or if desired a material which will decompose to form urea in the finished paper. However, because of economics, it will be preferable to use urea. The urea is introduced into the paper in the form of a concentrated water solution (usually up to about 50%). The concentrated urea solution may be introduced onto the wet sheet as it comes ofi the papermaking machine or between plies of wet sheets if they are to be made up into a multi-ply paperboard or after either the single sheet-s or the paperboards have been dried. It is, however, convenient to introduce the urea while the paper or paperboard is still wet since it is believed that better penetration is obtained in this manner. And, in fact, it is preferable that the urea be applied to each ply as the paper is being made to insure uniform internal treatment of the paper. In any event it is desirable to introduce the urea before the phenolic and urea-formaldehyde resins are cured such as by baking in a furnace or by baking the electrical equipment after it is assembled.

The urea is introduced into the paper in an amount to furnish between 1 and 7% by dry fiber weight of the urea in the final paper.

It is not completely understood why the addition of the urea to the resin-treated paper has the unexpected result that it does as shown in FIG. 1. However, it may be postulated that there are probably several reactions which may take place. First, it has been shown by Wiihler (see for example Textbook of Organic Chemistry, by George Holmes Richter, 2nd ed., 1946, John Wiley & Sons, Inc., pp. 231232) that urea in a water solution is in equilibrium with isocyanate radicals and it is possible that in this form it is capable of reacting with the cellulosic molecules since such reaction is recognized. Being introduced as a water solution the urea is capable of penetrating into the cellulosic fibers and it is possible that there it reacts with portions of the cellulosic molecule which have not already been reacted with or condensed with the phenolic or urea-formaldehyde resins. Finally, it is possible that some free formaldehyde may remain in the treated paper and the urea is available for condensation with this formaldehyde. Thus, it will be seen that the urea probably plays a multiple role and that each of the treating agents required and specified contributes not in a strict additive sense but in a truly synergistic effect as indicated in the very marked improvement in thermal stability achieved by the treatment of this invention shown in curve V of FIGS. 1 and 2.

Finally, a small amount of an elastomeric material may be added to the paper stock to act as a flocculating agent to aid in exhausting the resinous additives onto the fibers and to contribute an added degree of flexibility to the resulting paper. Such an elastomeric material may be a natural or synthetic rubber latex or a material such an an acrylonitrile-butadiene copolymer. Any elastomeric material which may be introduced into the water slurry of the cellulosic fibers and subsequently exhausted out onto the fibers is suitable for the practice of this invention. There is some reason to believe that the combination of an elastomeric material in a relatively small concentration with the phenolic resin adds flexibility, while not detracting from the thermal or dimensional stability attained. Such an elastomeric material may be added in an amount equivalent to from about 0.5 to 3.0% by weight of dry fiber in the finished paper.

In defining the treating agents which are suitable for this invention, and in specifying the amounts which are to be persent it has been noted that they are not interchangeable and that there is in fact a definite interrelationship between the amounts of each which may be present in the final paper. Thus, it is preferable that the ureaformaldehyde resins and the urea be present in essentially equivalent amounts although somewhat more ureaformaldehyde than urea may be used; while the phenolic resins should be prseent in an amount equivalent to from about 1 to times the amount of urea-formaldehyde resin or urea. A preferable ratio of phenolic resin to ureaformaldehyde resin to urea is 3:1:1, this being found to give a paper having optimum dimensional and thermal stability. The percentages of the treating agents in the paper are preferably 5% and 5% of phenolic resin, urea-formaldehyde resin and urea, respectively.

The process in general comprises the steps of forming a typical papermaking stock, adding to the stock the phenolic resin and urea-formaldehyde resin, and the elastomeric material usually in the form of a latex if required. In adding an elastomer as a latex dispersion to a large volume of water it is necessary to maintain the alkalinity to prevent the elastomer particles from agglomerating. This is done with ammonia. It is generally convenient to exhaust out the resins onto the fiber in keeping with good papermaking practice by lowering the pH of the stock to between 4.5 and 5.0. This is conveniently done by adding papermakers alum in sutficient quantity to reduce the pH to the desired level. In this process it is preferable, however, to achieve a slightly higher acidity than may be achieved with small amounts of alum and this is done by also adding some volatile acid such as acetic acid. The stock is then introduced into a papermaking machine which may be any of the known types such as a Fourdrinier or cylindrical machine. The resulting wet sheets may then be treated directly with the concentrated urea solution or may be dried over a drum dryer in accordance with known papermaking techniques and then treated with the concentrated urea solution. If a paperboard of a relatively thick material is to be formed, the wet sheets coming off the papermaking machine may be assembled to form a multi-ply board which then may be dried. In the case of the formation of a paperboard of this type the urea may be introduced onto the single wet sheets before assembly, between the sheets as they are assembled, or onto the assembled multi-ply sheets before they are treated. It may also be introduced into the paperboard after it has been dried.

The concentrated water solution of urea may be applied by spraying, brushing on, by immersing the sheet or paperboard to be treated in the urea solution, or any other known technique for introducing a liquid into and through paper.

Because of the assumed interaction of the urea with the resins and with the cellulosic fibers it is preferable to introduce the urea into the paper before the resins have been cured to their final irreversible stage. The paper may be cured by heating in any suitable fashion such as by placing in an oven using curing times and temperatures normally associated with the resins used. Typical times and temperatures are illustrated in Example I. Inasmuch as it is customary in the building of transformers to bake the entire electrical apparatus after assembly and before introduction of the oil, curing of the resins in the paper need not take place until this baking operation is performed.

The invention may be further described in the following example, which is meant to be illustrative and not limiting.

Example II A cellulose fiber slurry was made up as in Example I. Phenolformaldehyde as a water solution and a urea-formaldehyde dispersion were added to provide 12.5% and 2.5% by dry fiber weight of these resins respectively. After adjusting the pH to about 8.5 a natural rubber latex was added in an amount to furnish 2% solids based on dry fiber weight. After thorough mixing sufiicient papermakers alum and acetic acid were added to reduce the pH to 4- and the stock was introduced into a Fourdrinier machine to form sheets of paper. These sheets were plied while wet to form paperboard, which when dry would be /a inch thick. The multiple-sheet assembly was dried on a standard oven drier at a temperature below that at which any appreciable amount of the resins would be cured.

The dried paperboard was then immersed in a 50% water solution of urea until the urea solution had thoroughly penetrated it throughout and amounted to 2.5 solids based on the dry fiber weight. The urea-containing board was then dried and baked at 350 F. for 2 hours to cure the resins and accomplish whatever irreversible reactions take place within the board. The resulting paperboard exhibited thermal stabilities and dimensional stabilities comparable to the paper of Example I above.

In these examples, the cellulosic fibers used were a mixture of rag and kraft. The process of this invention is equally adaptable to all rag, or all kraft, or any mixture thereof.

It will be seen from the above description of this invention, and from the examples and drawings that there is provided a process for materially improving the thermal and dimensional stability of a cellulosic sheet or board and which in turn provides a paper which offers the possibility of attaining improved operating conditions in electrical apparatus. The cellulosic material thus formed may, of course, also find widespread use in other applications, its use in electrical apparatus being given as illustrative of the high degree of utility which the cellulosic material of this invention possesses.

We claim:

1. Process for treating cellulosic material to render it dimensionally and thermally stable, characterized by the step of incorporating into sheets formed of cellulosic fibers from 5 to 30% by dry fiber weight of an uncured phenolic resin, from 2 to 10% by weight of an uncured amineformaldehyde resin prior to web formation and from 1 to 7% by weight urea subsequent to web formation.

2. Process in accordance with claim 1 wherein said amine-formaldehyde resin and said urea are added in approximately equal amounts and said phenolic resin is added in an amount equivalent to from 1 to 5 times the amount of said urea.

3. Process for forming a cellulosic sheet from a water slurry including the steps of forming a papermaking stock and sheeting out said stock, characterized by the step of adding to said paper stock from 5 to 30% by dry fiber weight of a phenolic resin and from 2 to 10% by weight of a urea-formaldehyde resin, treating the paper formed therefrom with a concentrated water solution of urea in an amount sufiicient to incorporate in said paper from about 1 to 7% urea by dry fiber weight and subsequently curing said resins.

4. Process for forming a cellulosic sheet from a water slurry, comprising the steps of forming a stock of cellulose fibers, adding to said stock from 5 to 30% by dry fiber weight of a phenolic resin, from 2 to 10% by weight of a urea-formaldehyde resin and up to 3.0% of an elestomeric material, forming paper from said stock, drying the resulting paper, treating said paper with a concentrated solution of urea in an amount sufficient to incorporate in said paper from about 1 to 7% by weight urea based on dry fiber weight, and curing said paper whereby said resins and said urea are caused to interact and to react with said cellulose fibers.

5. Process in accordance with claim 4 wherein said treating step is accomplished prior to said drying.

6. Process in accordance with claim 4 wherein said treating step is accomplished subsequent to said drying.

7. Process for forming a cellulosic material which has low hygroexpansivity and is resistant to heat, comprising the steps of forming a stock of cellulosic fibers, adding to said stock from to 30% by dry fiber weight of a phenolic resin and from 2 to of a urea-formaldehyde resin, forming paper from said stock, spraying a concentrated water solution of urea on said paper while wet to form a urea-containing sheet, and drying and curing said sheet, the amount of urea being introduced being equivalent from about 1 to 7% by dry fiber weight.

8. Process in accordance with claim 7 further characterized by the step of building up a plurality of said sheets into an assembly thereby to form a multi-ply paperboard prior to said drying step.

9. Process for forming a cellulosic material which has low hygroexpansivity and is resistant to heat, comprising the steps of forming a stock of cellulose fibers, adding to said stock from 5 to 30% by dry fiber weight of a phenolic resin, and from 2 to 10% of a urea-formaldehyde resin and from 0.5 to 3.0% of an elastomeric material, forming paper from said stock, forming a multi-ply paperboard prior to drying said paper, impregnating said paperboard with a concentrated water solution of urea and drying and curing the resulting urea-impregnated paperboard, said urea being present in said paperboard in an amount equivalent to from about 1 to 7% by dry weight of said paperboard.

10. Process in accordance with claim 9 wherein said impregnating step is accomplished subsequent to said drying step.

11. A paper formed from cellulosic fibers having low 10 hygroexpansivity and high resistance to heat, characterized as containing therein from 5 to 30% by dry fiber weight of a phenolic resin, from 2 to 10% of a urea-formaldehyde resin and from 1 to 7% urea.

12. A sheet in accordance with claim 11 further characterized by containing up to 3.0% of an elastomeric material.

13. A sheet in accordance with claim 11 wherein said urea-formaldehyde resin and said urea are present in substantially equivalent amounts and said phenolic resin is present in an amount equivalent to from 1 to 5 times the amount of urea.

14. Sheet in accordance with claim 11 wherein said phenolic resin, said urea-formaldehyde resin and urea are present in amounts equivalent to about 15%, 5% and 5%, respectively, by dry fiber weight.

15. A paperboard formed of a plurality of interbonded cellulosic sheets, characterized as containing in addition to the cellulosic fibers used in forming said sheets from 5 to 30% by dry fiber weight of the phenolic resin, from 2 to 10% of a urea-formaldehyde resin and from 1 to 7% of urea.

References Cited in the file of this patent UNITED STATES PATENTS 1,953,832 Sandell Apr. 3, 1934 2,303,436 Little Dec. 1, 1942 2,309,089 Bauer et a1. Ian. 26, 1943 2,563,897 Wilson Aug. 14, 1951 

4. PROCESS FOR FORMING A CELLULOSIC SHEET FROM A WATER SLURRY, COMPRISING THE STEPS OF FORMING A STOCK OF CELLULOSE FIBERS, ADDING TO SAID STOCK FROM 5 TO 30% BY DRY FIBER WEIGHT OF A PHENOLIC RESIN, FROM 2 TO 10% BY WEIGHT OF A UREA-FORMALDEHYDE RESIN AND UP TO 3.0% OF AN ELASTOMERIC MATERIAL, FORMING PAPER FROM STOCK, DRYING THE RESULTING PAPER, TREATING SAID PAPER WITH A CONCENTRATED SOLUTION 